1. Field of the Invention
This invention relates to a variable turbine geometry turbocharger for an internal combustion engine. More particularly, this invention relates to a variable turbine geometry turbocharger having adjustable guide vanes with a variable pivot center.
2. Description of Related Art
A turbocharger is a type of forced induction system used with internal combustion engines. Turbochargers deliver compressed air to an engine intake, allowing more fuel to be combusted, thus boosting an engine's power density without significantly increasing engine weight. Thus, turbochargers permit the use of smaller engines that develop the same amount of power as larger, normally aspirated engines. Using a smaller engine in a vehicle has the desired effect of decreasing the mass of the vehicle, increasing performance, and enhancing fuel economy. Moreover, the use of turbochargers permits more complete combustion of the fuel delivered to the engine, which contributes to the highly desirable goal of reduced emissions.
Turbochargers include a turbine having a turbine housing connected to the engine's exhaust manifold, a compressor having a compressor housing connected to the engine's intake manifold, and a bearing housing connecting the turbine and compressor housings together. The turbine includes a turbine wheel disposed within the turbine housing and the compressor includes a compressor impeller disposed within the compressor housing. The turbine wheel is rotatably driven by a flow of exhaust gas supplied from the exhaust manifold. A shaft is rotatably supported in the bearing housing and couples the turbine wheel to the compressor impeller such that rotation of the turbine wheel causes rotation of the compressor impeller. The shaft connecting the turbine wheel and the compressor impeller defines an axis of rotation. As the compressor impeller rotates, it compresses ambient air entering the compressor housing, thereby increasing the air mass flow rate, airflow density, and air pressure delivered to the engine's cylinders via the engine's intake manifold.
To improve efficiency, responsiveness, or the operating range of turbochargers, it is often advantageous to regulate the flow of exhaust gas to the turbine wheel. One method of regulating the flow of exhaust gas to the turbine wheel is commonly referred to by several names, including Variable Turbine Geometry (VTG), Variable Geometry Turbine (VGT), Variable Nozzle Turbine (VNT), or simply Variable Geometry (VG). VTG turbochargers include a plurality of adjustable guide vanes pivotally supported within a wheel inlet leading to the turbine wheel. The space between adjacent guide vanes constitutes flow channels for regulating the flow of exhaust gas to the turbine wheel. The geometry of the flow channels is adjustable by pivoting the guide vanes within a pre-determined range of angular positions between an open position and a closed position. In the open position, the guide vanes are oriented in a generally radially extending position relative to the axis of rotation of the shaft to allow the flow of exhaust gas through the wheel inlet to the turbine wheel. In the closed position, the guide vanes are oriented in a generally tangentially extending position relative to the axis of rotation of the shaft to block the flow of exhaust gas to the turbine wheel.
To control the boost pressure delivered to the engine, the guide vanes are adjusted to constrict or open the flow channels between adjacent guide vanes. Constricting the flow channels increases the velocity of the exhaust gas impacting the turbine wheel, which causes the turbine wheel to rotate more quickly. Increasing the rotation of the turbine wheel in turn increases the rotation of the compressor impeller, and thereby increases the boost pressure delivered to the engine. Conversely, opening the flow channels decreases the velocity of the exhaust gas impacting the turbine wheel, which causes the turbine wheel to rotate more slowly. Decreasing the rotation of the turbine wheel in turn decreases the rotation of the compressor impeller, and thereby decreases the boost pressure delivered to the engine. The guide vanes also provide a means for controlling and generating exhaust gas back pressure in engines which use Exhaust Gas Recirculation (EGR) to control Nitrogen Oxide (NOx) emissions.
Typically, the guide vanes pivot between the open and closed positions about a fixed pivot post. The pivot post for each guide vane is positioned between a leading edge and a trailing edge of the respective guide vane. When the guide vanes are in the open position it is aerodynamically advantageous to have the pivot post positioned towards the leading edge. This results in a stable aerodynamic flow of the exhaust gas through the flow channels and prevents destructive vane flutter from occurring. However, if the pivot post is positioned towards the leading edge when the guide vanes are in the closed position, the exhaust gas creates a pressure delta forward and rearward of the pivot post that tends to urge the guide vanes to pivot towards the open position. As such, an actuation effort that is undesirably high is required to maintain the guide vanes in the closed position. In contrast, if the pivot post is positioned generally midway between the leading and trailing edges when the guide vanes are in the closed position, the pressure delta forward and rearward of the pivot post is generally equalized.
It is desirable, therefore, to provide a variable turbine geometry turbocharger including adjustable guide vanes having a pivot location which varies as the guide vanes pivot between an open position and a closed position. It is further desirable that the guide vanes pivot about a pivot post that is positioned towards a leading edge when the guide vanes are in the open position and is positioned generally midway between the leading edge and a trailing edge when the guide vanes are in a closed position.